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May 7, 2010 -- Taking your vitamin D supplement with the largest meal of the day may boost its absorption substantially, according to a new study.

Researchers from the Cleveland Clinic instructed 17 men and women, average age 64, whose blood levels of vitamin D were borderline insufficient despite taking supplements, to take their supplements with the largest meal of the day.

After two or three months, the study participants had about a 50% increase in blood levels of the vitamin, regardless of the dose they took.

Researchers Guy B. Mulligan, MD, and Angelo Licata, MD, had noticed that patients typically report taking the supplement either on an empty stomach or with a light meal.

Because the vitamin is fat-soluble, the researchers speculated that taking it with a big meal would improve absorption.

Vitamin D is crucial not only to maintain bone strength, but research now suggests it plays a role in immune system problems, cancer, and cardiovascular disease.

The researchers measured blood levels of the vitamin at the start of the study and two or three months later. Participants took a range of doses, and the researchers divided them into three groups: less than 50,000 IU a week, 50,000 IU, and more than 50,000 IU. The daily doses ranged from 1,000 IU to 50,000 IU.

A dose of 400 IU is termed adequate for people 51-70, and 600 IU for people 71 and older, as set by the Institute of Medicine, but some experts believe much more is needed, especially in older adults. The current upper tolerable level is set at 2,000 IU daily. The recommendations are under review and an update is expected this month.

At the study start, the average blood level of the form of vitamin D measured, 25(OH)D, was 30.5 nanograms per milliliter. By the end, it was 47.2 ng/mL. A level of 15 and higher is termed adequate by the Institute of Medicine for healthy people, but the study participants had a range of health problems, such as osteoporosis and thyroid problems.

Few foods contain vitamin D naturally, and some foods are fortified with it. Vitamin D synthesis is also triggered when the body is exposed to sunlight.

The research is published in the Journal of Bone and Mineral Research.